The invention relates generally to power supplies, and, more particularly, to diode recovery current suppression circuits for use with power supplies.
Most modern power supplies employ fast switching diodes to rectify the supplied output voltage. Diodes, of course, have two states, namely, an on state and an off state. In the on state a voltage is applied across the diode in the forward direction. In the off state, a reverse voltage is applied and the diode is intended to act as an open switch (i.e., no current flow).
Modern high frequency switching power supplies alternate between the on and off states very quickly. This quick alternation leads to a high change in voltage as a function of time (dv/dt) in the time period between the on and off states of the diode. This high dv/dt causes a reverse current flow (i.e., diode reverse recovery current) through the diode for a short period of time after the reverse voltage is applied across the diode.
The diode reverse recovery current is usually manifested by a high reverse current spike which can be several times higher than the forward current. This current spike increases converter EMI and converter losses. The time length of this current spike is usually 30-100 ns, or even more if low speed diodes are used. This short spike is known to be one of the main EMI sources in almost every power supply. It limits the switching frequency of the power supply and sometimes forces the designer to reduce the switching speed in order to reduce EMI, which also leads to lower efficiency.
There are many known methods for diode recovery current suppression. Most of these methods have limited application (i.e., they can work only on certain types of power converters). Other known solutions work only in cases of certain input and/or output conditions. Many known solutions involve complex circuits and active devices.
One prior art solution to diode recovery current suppression is illustrated in Carsten, U.S. Pat. No. 5,307,004. Carsten proposes the use of an auxiliary inductor (L2), an auxiliary diode (D2) and a second switch (Q2) to suppress reverse recovery current in a main diode (D1) of a boost regulator or a buck regulator. In either circuit arrangement, the auxiliary inductor (L2) is coupled to a tap on a main inductor (L1) so that one end of the auxiliary inductor (L2) is at a lower potential than the input to the main diode (D1). This circuit topology necessitates the inclusion of the second switch (Q2) to achieve current suppression.